Imaging devices, including charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) imagers, are commonly used in photo-imaging applications.
A CMOS imager circuit includes a focal plane array of pixels, each one of the cells including a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. Each pixel has a readout circuit that includes at least an output field effect transistor formed in the substrate and a charge storage region formed on the substrate connected to the gate of an output transistor. The charge storage region may be constructed as a floating diffusion region. Each pixel may include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference.
In a CMOS imager, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region accompanied by charge amplification; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
CMOS imagers of the type discussed above are generally known as discussed, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524 and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc., which are hereby incorporated by reference in their entirety.
Relatively long signal lines are used in an imager integrated circuit to implement and interconnect the components used to process image data. A tree type routing scheme is widely accepted in many long signal line applications, including imager applications, where timing convergence, automatic placement and routability are required. Typically, automatic design tools use the tree type routing scheme to achieve the best possible maximum operating frequency. These design tools insert many line buffers into the design to meet the timing constraints and maintain signal integrity. However, the tree type routing scheme does not work well when implemented in large/small aspect ratio (i.e., width/height ratio of an array, for example an imager array or a memory block) designs. Often, too many line buffers are inserted into the design, which results in the timing constraints not being met, which in turn results in poor performance.
Typically when a long line is routed, the tree type routing scheme inserts a repeat line buffer corresponding to a first buffer branch 121, located in a first stage 217, that is at the center of the target signal sink points (e.g., device inputs 1-16), as illustrated in FIG. 1. FIG. 1 also illustrates three more buffer stages 218, 219, 220 and many more branches 122, 123, 124. In a branch subsequent to the first branch 121, repeat line buffers are placed at the halfway point of the span between the left or right terminal and the repeat line buffer of the first branch 121 (or other previously placed buffers). This process continues until the repeat line buffers are close enough to the sink points (circuit inputs) 1-16 and the loading capacitance is small enough to be driven.
The tree type routing scheme is an effective scheme when deriving the maximum operational frequency of the whole circuit having an aspect ratio of approximately 1:1. In this implementation, the operating frequency as a whole is not hindered by several extremely slow paths. However, this routing scheme is not effective when the aspect ratio is large/small.
In the tree type routing scheme the sink points are driven by the last stage line buffers (e.g., line buffers 125, 126, 127, 128, 129, 130, 131, 132). The tree type routing scheme is arranged such that all of the sink points 1-16 (circuit inputs) are driven and switched at approximately the same time, which results in a high current peak at the switching point. This is often undesirable. Further, in the tree type routing scheme many horizontal signal lines are required because the direction of the signals is bidirectional in the horizontal direction of the routing scheme illustrated in FIG. 1. This characteristic may degrade the routing metal and thus create a higher risk of area congestion.
Accordingly, there is a need and desire for a buffering technique that reduces the number of line buffers and provides a propagation delay which reduces the peak current.